In the conventional way, the master cylinder is full of brake fluid and equipped with a main hydraulic piston intended to receive an actuating force composed of an input force and of a boost force both acting in an axial direction.
On the other hand, the pneumatic booster can be controlled by application of the input force to a control rod controlling the opening of a valve so as to exert the actuating force on the main hydraulic piston of the master cylinder, the booster including a rigid casing divided in leaktight fashion into two chambers by means of a moving partition which can be acted upon by a difference in pressure between the two chambers resulting from the opening of the valve and drive along a pneumatic piston which can move with respect to the casing and which carries the valve, the input force being transmitted via a reaction disc against which the pneumatic piston also rests in order to impart at least some of the boost force thereto.
A device of this type is well known in the prior art and is described, for example, in document U.S. Pat. No. 4,491,058.
These braking devices have, as advantage, as a result of the use of a pneumatic piston which can move with respect to the rigid casing, the fact that the total travel available to the control rod, and therefore to the brake pedal, is relatively long, which constitutes a necessary condition in providing optimum control over the brake fluid pressure supplied to the brakes of the vehicle.
In parallel, boosted braking devices have been developed in which the reaction exerted on the control rod is supplied no longer mechanically by a reaction disc, but by the hydraulic pressure prevailing in the master cylinder.
In these devices, the main hydraulic piston of the master cylinder itself includes a hollow moving cylinder communicating with the master cylinder, receiving at least some of the boost force, and inside which there slides, in leaktight fashion and in the axial direction, a reaction piston which can receive at least the input force, elastic means exerting an elastic force between the reaction piston and the moving cylinder and urging the reaction piston towards the master cylinder, at least one opening being made in the moving cylinder to make the inside thereof communicate with the inside of the master cylinder.
Such a device is described, for example, in document FR-A-2,658,466.
These devices with hydraulic reaction have the main advantage that irrespective of the intensity of the braking action or the rate of application of the input force, their characteristic operating curve, namely the curve giving the pressure in the master cylinder as a function of the intensity of the input force on the booster, is unchanged.
However, all these boosted braking devices, whether they have mechanical reaction or hydraulic reaction, allow only the hydraulic pressure supplied to the wheel brakes to be controlled, but do not allow all the parameters involved in the braking of a motor vehicle to be taken into account.
One important parameter involved in braking is the coefficient of adherence between the wheel to be braked and the ground over which it is running. During a braking action, this coefficient can vary greatly, for example if the vehicle hits a portion of wet ground while the driver is braking on dry ground. As the adherence of the wheels to the ground in such a case is then appreciably lower, there is the risk of the wheels locking and the vehicle skidding.